Surgical heart valve replacement is usually carried out on a cardioplegic (arrested) heart. The affected heart valve is cut out and the heart valve prosthesis to be implanted is stitched in. This complicated procedure is carried out with the aid of heart-lung machines and is to some extent not an option for high-risk patients. Thus, a minimally invasive or percutaneous method to replace a heart valve is sought after, so that this cardiosurgical intervention can routinely be offered to a broad group of patients in the long-term.
Depending on the anatomical and physiological condition of the patient (e.g. condition of the femoral arteries), two implantation methods are available for the physician—transfemoral implantation (via the groins) and transapical implantation (via the cardiac apex). Each strategy requires a respective adapted application device since the valve stent one case is ejected against flow direction (retrograde) and in the other case with flow direction (anterograde). During the implantation process, the valve prosthesis must be easily maneuverable through the access vessels and at the implantation site to ensure optimal position accuracy. Furthermore, the prosthesis must be able to be securely anchored. Amongst other things, a certain excess in the diameter of the valve prosthesis compared to the blood vessel is generally used in order to generate a radial pressure on the blood vessel by the prosthesis.
These percutaneous applicable valve systems consist of a valve-carrying stent, additional fixing elements connected to this stent (and, if necessary, further structures) to anchor the heart valves in the heart and in the vessel of the patient and an application device (or catheter system) for minimally invasive introduction and positioning of the heart valve at the implantation site in the patient.
In principle, such systems can also be used to replace defective venous valves or to relieve them by connecting them in series with an artificial venous valve. Unlike heart valve prosthesis, venous valve prosthesis can also be implanted heterotopically (not orthotopically), i.e. not at the same place as the body's own venous valve.
Usually, conventional stent designs are similar to an enlarged but valve-carrying coronary stent or a stent with sawtooth-, diamond- or meander-shaped netlike braid to brace vasoconstrictions. The stents are thereby formed as self-expanding or not self-expanding, foldable structures often without special fixing systems. However, these stents are not or only insufficiently adapted to the biomechanics and anatomy of highly stressed heart- or venous valve prostheses.
Another stent is disclosed in DE 20 2007 005491 U1. The medical device described therein serves for treatment of aortic valve insufficiency using a self-expandable endoprosthesis that can be introduced minimally invasively into the patient's body to position and fix a heart valve prosthesis in the aorta of the patient, wherein the endoprosthesis has at least three position brackets for independent positioning of the medical device in the aorta of the patient and one retaining segment with retaining brackets to take up a heart valve prosthesis, and wherein, during introduction of the medical device into the body of the patient, the endoprosthesis has a shaping that can be defined in advance, wherein the medical device has a folded state in the initial shaping of the endoprosthesis and an expanded state in the second shaping of the endoprosthesis. It is also provided that the endoprosthesis is in one piece with a structure cut from a metal tube in which any position bracket is assigned to one retaining bracket and in which, at the distal end of the endoprosthesis, any end portion of the respective position bracket is connected to the associated retaining bracket.
Shape memory materials are used for the self-expanding valve-carrying stent systems and, if necessary, for additional anchor and fixing elements. These can be shape memory polymers or memory metals, such as copper/zinc/aluminium-, copper/aluminium/nickel or nickel/titanium (“nitinol”) shape memory alloys. In particular, shape memory materials with one-way effect are used, where the alloy only returns to the former shape when the critical temperature or the shape transformation temperature is exceeded and retains this shape even if the temperature once again falls below the critical temperature. Memory metals are usually chosen such that the critical temperature is below the body temperature, so that the expansion into the final shape is evoked at body temperature, i.e. during introduction of the valve prosthesis into the patient. The non-self-expanding, valve-carrying stents are mostly made of stainless steel alloys, such as 316 L. Spring steel, such as Phynax/Elgiloy or high strength polymers or plastics, such as PEEK, polyurethane and fibre- or nano-reinforced polyurethanes, are also suitable for valve, bulb or anchoring stents, as well as for vascular stents and stent grafts. Alternatively, they can also be made of bio-resorbing materials, for example based on magnesium alloys or bio-resorbable plastics (such as polylactate or other specific polyurethanes).
The previously described stents are disadvantageous as a result of their stiffness under forces directed radially inwards. Although it is possible in principle to increase the stiffness of a stent using more supporting material, this results in an adverse increase of the area of extraneous material within the blood vessels. This increases the risk of thrombotic accumulations, for example.